A thermal processing chamber as used herein, refers to a device that rapidly heats objects, such as semiconductor wafers. Such devices typically include a substrate holder for holding a semiconductor wafer and a light source that emits light energy for heating the wafer. During heat treatment, the semiconductor wafers are heated under controlled conditions according to a preset temperature regime. For monitoring the temperature of the semiconductor wafer during heat treatment, thermal processing chambers also typically include radiation sensing devices, such as pyrometers, that sense the radiation being emitted by the semiconductor wafer at a selected wavelength. By sensing the thermal radiation being emitted by the wafer, the temperature of the wafer can be calculated with reasonable accuracy.
Many semiconductor heating processes require a wafer to be heated to high temperatures so that various chemical and physical transformations can take place as the wafer is fabricated into a device. During rapid thermal processing, which is one type of processing, semiconductor wafers are typically heated by an array of lights to temperatures, for instance, from about 400.degree. C. to about 1,200.degree. C., for times which are typically less than a few minutes. During these processes, one main goal is to heat the wafers as uniformly as possible.
Problems have been experienced in the past, however, in being able to maintain a constant temperature throughout the wafer due to heat loss that occurs at the edge of the wafer. For instance, due to the increased surface area to volume ratio, the edges of semiconductor wafers tend to lose more heat by radiation than the surfaces of the wafer. Many thermal processing chambers are also constructed such that the central region of the wafer is surrounded by highly reflective surfaces, while the edges of the wafer face less reflective surfaces. This arrangement causes the wafer to heat up nonuniformly and for the edges of the wafer to have an increased tendency to loose heat in comparison to the top and the bottom of the wafer.
The above problems with heat loss through the edges of the wafer can further be compounded if the wafer is exposed to a flow of gas during heat treatment. In particular, when the wafer is exposed to a flow of gas, increased convective cooling may occur at the edges of the wafer. Ultimately, these energy losses can create different temperatures zones within the wafer during heat treatment, which adversely impacts upon the ability of the process to produce precise and uniform semiconductor devices.
Currently, in order to diminish the effect of heat losses, passively heated slip-free rings are sometimes placed adjacent to the edges of the wafer. For instance, slip-free rings, which are usually made from silicon or silicon carbide, are designed to surround the wafer so that the wafer is positioned within the inside diameter of the ring. During operation of the thermal processing chamber, the ring absorbs energy being emitted by the heater lamps and then radiates the energy back to the edges of the wafer. The ring can also reflect thermal radiation being emitted by the edge of the wafer back onto the wafer. In this manner, the passively heated ring compensates for heat loss at the edge of the wafer and also reduces convective cooling of the wafer if process gases are flowing through the chamber.
Although providing improvements in the ability to more uniformly heat semiconductor wafers, passive slip-free rings used in the past do present a number of limitations and disadvantages. For instance, the slip-free rings can absorb large amounts of energy from the heat source that is used to heat the wafers, thus increasing energy demands for the system. Also, because the slip-free rings are passively heated, the rings must be designed to effectively absorb heat. Consequently, the rings can only have a limited number of shapes.
Another problem that has been experienced in the past with slip-free rings is the ability to maintain the ring in alignment with a semiconductor wafer that is being heated. Further, passively heated rings offer little control over the rate at which the rings are heated or cooled. It would be desirable if the temperature of the rings could be controlled in conjunction with a heat treating process.
In this regard, various attempts have been made in the past to design heating elements that are heated by electrical resistance and shaped to surround the wafer. For instance, such heating elements are disclosed in U.S. Pat. No. 4,469,529 to Mimura, U.S. Pat. No. 4,493,977 to Arai, et al., U.S. Pat. No. 4,535,227 to Shimizu and U.S. Pat. No. 4,535,228 to Mimura, et al.. In these references, the heating elements include a filament, such as made from a metal, that is surrounded by a tube made of, for instance, silica glass.
Further improvements in methods and devices for compensating for heat loss at a wafer's edge, however, are still needed. As will be made apparent from the following description, the present invention is directed to further improvements in such devices.